This invention relates to the art of using welding techniques to rebuild the interior walls of hollow workpieces and has special utility for rebuilding the inner walls of the shells of centrifugal dredge pumps. Such pump shells have discharge ports of 14 inches diameter or larger with two opposite concentric door openings of 48 inches or larger on the axes of the shells, the shells being several feet in diameter and weighing several tons. The inner walls of the pump shells are severely abraded by the dredged material but are commonly restored by the deposition of weld metal, preferably by means of an electric welding torch. The problem has been to develop an apparatus to carry out such a welding procedure as nearly automatically as possible.
For this purpose the Braucht U.S. Pat. No. 3,254,192 discloses a boom that is mounted directly on a pump shell that is to be processed. The boom carries a torch inside the pump shell and the operator makes manual adjustments as necessary for radial extension and retraction of the traveling torch and makes vertical adjustments to step over from one inner circumferential welding traverse to the succeeding inner circumferential welding traverse. When the pump shell is turned over to prepare for welding the second half of the shell, the boom must be removed from the pump shell and must then be mounted again on the second half of the pump shell.
The Smith U.S. Pat. No. 3,627,973 discloses how a curved guide of spiral configuration may be mounted directly on a volute-type pump shell to guide a carriage inside the shell along a required eccentric or spiral path. The rotary direction of the torch travel is automatically reversed at the end of each inner circumferential traverse and step-over Jo the next succeeding inner circumferential traverse is automatic. To prepare for welding the second half of the pump shell, the curved guide must be dismantled and then mounted on the second half of the pump shell. The guide is in the form of a metal band that is bent by hand to the desired curvature and therefore does not conform accurately to the desired curvature. The band is supported at spaced points and may be flat instead of curved between the spaced support points.
In both of the above prior art disclosures the power-driven mechanism for feeding welding wire to the torch is in fixed position separate and apart from the turning structure that swings the welding torch circumferentially inside the pump shell over a range of as much as 360.degree.. Consequently, the wire drive mechanism is remote from the torch and the flexible welding wire conduit that feeds the torch must twist to follow the wide arc of the torch. Another disadvantage is that the loop of welding wire conduit that leads to the torch hampers the operator and limits his access to the interior of the pump shell. In addition, the loop of conduit obstructs the use of a water cooling ring which is commonly used to trickle cooling water over the outside of the pump shell.
Also, in both of the prior disclosures the entire turning structure is connected to the ground side of the welding circuit. In the Smith disclosure the gearing that controls the tilt of the torch is included in the grounded structure.
In the Smith disclosure, at the end of each circumferential traverse the torch immediately steps over and reverses direction to start the next traverse. As a result weld metal at the beginning of the next traverse is deposited contiguous to liquid metal at the end of the first traverse and the two liquid deposits merge to form a single liquid body of such quantity as to tend to flow gravitationally to produce what is termed a dribble. This undesirable action occurs until the torch reaches the region of solidified metal of the first traverse.
In the rebuilding of the interior of a pump shell the operator always has to deal with local regions where the shell wall is deeply eroded to form low spots or craters. Each of the low spots must be built up before the welding torch can be employed for complete circumferential traverses of the shell interior. To build up a local low spot, the Smith apparatus is programmed to cause the torch to repeatedly reciprocate across the low spot with no loss of time at each automatic step-over and reverse in travel. Since the low spot is of limited width liquid metal is added to liquid metal on each traverse.
A further difficulty in building up a local low area may be understood when it is considered that in making a full circumferential traverse the torch travels many feet over a substantial time period so that the heat from the torch is effectively dissipated without any undue tendency to distort the pump shell. In repeatedly reciprocating the torch across a relatively narrow low area, however, the heat of the torch is concentrated in the low area with inevitable undesirable results.
In rebuilding the shell of a volute-type pump, a crescent-shaped area bordering on each circular door must be processed and the most efficient coverage would be by a pattern of spiral beads terminating at spaced points along the rim of the door opening. The Smith disclosure which provides for automatic step-over and automatic travel reversal cannot be programmed to cover the crescent-shaped area with the desired pattern of weld beads.